A search for massive ultra-compact dwarf galaxiesin the Centaurus galaxy cluster

摘要

Context. In a previous paper, we initiated a search for ultra-compact dwarf galaxies (UCDs) in the Centaurus galaxy cluster, resultingin the discovery of 27 compact objects with –12.2 < M_V < –10.9mag. Our overall survey completeness was 15-20 within 120 kpcprojected clustercentric distance.Aims. To constrain the luminosity distribution of the brightest UCDs in Centaurus in a more accurate way, we continue our search byimproving substantially our survey completeness specifically for brightnesses of M_V <–12 mag (V_0< 21.3 mag).Methods. Using VIMOS at the VLT, we obtained low resolution spectra of 400 compact objects with 19.3 < V_0< 21.3 mag(-14 < M_V < –12 mag at the Centaurus distance) in the central 25' of the Centaurus cluster, which corresponds to a projected radiusof –150 kpc. Our survey had complete area coverage within a radius of –120 kpc.Results. For 94 of the sources included in the masks, we measure successfully a redshift. Because of incompleteness in the slitassignment, our final completeness in the area surveyed is 52. Among our targets, we find three new UCDs in the magnitude range–12.2 < M_V <–12 mag, hence at the faint limit of our survey. For one these targets HST WFPC2 imaging is available, yielding asize estimate of r_h≤8-9 pc. At 95 confidence, we can reject the hypothesis that in area surveyed there are more than 2 massiveUCDs with M_V < –12.2 mag andr_(eff)≤70 pc. Hence our survey confirms the extreme rarity of massive UCDs. We find that the radialdistributions of Centaurus and Fornax UCDs with respect to their host clusters' centres agree within the 2σ level.